Improve error handling in BroadcastTimeDefs::string_to_date() (#745)

* Improve the error handling and/or recovery of BroadcastTimeDefs::string_to_date().

* Fix tests.

* Temove trivial white space.

* Fix review comments.

src/openlcb/BroadcastTimeDefs.cxx

@@ -189,11 +189,32 @@ int16_t BroadcastTimeDefs::string_to_rate_quarters(const std::string &srate)
 bool BroadcastTimeDefs::string_to_date(
     const std::string &sdate, int *year, int *month, int *day)
 {
-    struct tm tm = {};
+    struct tm tm;
+    memset(&tm, 0, sizeof(tm));
     if (strptime(sdate.c_str(), "%b %e, %Y", &tm) == nullptr)
     {
         return false;
     }
+
+    // newlib does not have the proper boundary checking for strptime().
+    // Therefore we use mktime() to determine if the time we have is really
+    // valid or not. In newlib, mktime() can actually correct some invalid
+    // struct tm values by making some educated guesses.
+    //
+    // While glibc does have proper boundary checking for strptime(), it
+    // can still use mktime() to correct some invalid struct tm values by
+    // making some educated guesses.
+    time_t t = mktime(&tm);
+
+    // newlib does not correctly set the errno value when mktime()
+    // encounters an error. Instead it "only" returns -1, which is technically
+    // a valid time. We are counting on the fact that we zeroed out the struct
+    // tm above, and subsequently -1 cannot be an expected result.
+    if (t == (time_t)-1)
+    {
+        return false;
+    }
+
     if (tm.tm_year < (0 - 1900) || tm.tm_year > (4095 - 1900))
     {
         // Out of range for openlcb.

src/openlcb/BroadcastTimeDefs.cxxtest

@@ -165,9 +165,10 @@ TEST_F(BroadcastTimeDefsTest, string_to_date)
     parse_date("Feb 29, 2024", 2024, 2, 29);
 
     // Not leap year
-    parse_date("Feb 29, 2023", 2023, 2, 29);
-    // No canonicalization in glibc
-    parse_date("Feb 31, 2023", 2023, 2, 31);
+    parse_date("Feb 29, 2023", 2023, 3, 1);
+
+    // canonize
+    parse_date("Feb 31, 2023", 2023, 3, 3);
 }
 
 TEST_F(BroadcastTimeDefsTest, string_to_date_fail)
@@ -202,9 +203,9 @@ TEST_F(BroadcastTimeDefsTest, date_canonicalize)
 
     EXPECT_FALSE(canon_date("Aug 12, 2023"));
     EXPECT_TRUE(canon_date("Aug     12, \n\t   2023", "Aug 12, 2023"));
-    EXPECT_FALSE(canon_date("Feb 31, 2023"));
-    EXPECT_FALSE(canon_date("Feb 31, 23"));
-    EXPECT_FALSE(canon_date("Feb 31, 3023"));
+    EXPECT_TRUE(canon_date("Feb 31, 2023", "Mar  3, 2023"));
+    EXPECT_TRUE(canon_date("Feb 31, 23", "Mar  3, 23"));
+    EXPECT_TRUE(canon_date("Feb 31, 3023", "Mar  3, 3023"));
 
     EXPECT_TRUE(canon_date("Aug 9, 2023", "Aug  9, 2023"));